investigators say the blast that collapsed the lower floors of the pemex building in mexico city yesterday. appears to have been accidental. but all possibilities are being examined. more than 120 were injured in the blast. a government official says early signs point to a problem in an area that housed electrical and air- conditioning equipment. >> nasa is marking 10-years since the columbia disaster. with a ceremony honoring all the astronauts who gave their lives for space exploration. on february first, 2003, the space shuttle columbia broke up upon re-entering the earth's atmosphere, killing all seven astronauts on board. today, dozens gathered at the space mirror memorial, the 42-foot high black granite memorial at kennedy space center in florida. the memorial contains the names of the 24-people who were killed on missions and other training and commercial accidents. former astronaut buzz aldrin also helped lead a tribute to fallen astronauts at arlington national cemetery. former massachusetts senator and one-time presidential candidate john kerry is now officially the country's sec